INTRODUCTION

[Pages:9]Recovered Paper Yield Estimate Using Laboratory and Mill Accounting Data

Robert de Jong, President, 2Fiber Consulting

ABSTRACT

Yield is an important process parameter with serious economic and environmental consequences. Precise calculation of yield is not easy. Mills use various approaches to estimate yield. The objective of this paper is to compare the yield for each furnish component with adjustment for actual process losses by combining paper machine product data, monthly mill accounting data, and data for each furnish component. Laboratory data for % moisture, 6 cut rejects, solubles, fines + ash are included in an example for a furnish of Newsprint, Sorted White Ledger, Coated Book, wet lap DIP, and mill broke. The model can be expanded to assign bleach chemicals and residue disposal tons to the appropriate furnish component. Tracking yield results will help identify economically and environmentally attractive furnish components for the mill.

INTRODUCTION

When recovered paper is received by the mill, their objective is to get as much of it as possible to the paper machine headbox without affecting product quality. However, certain losses are inevitable. Components of the furnish which contribute to a decrease in yield include:

? Contaminants such as staples, paper clips, plastics, wet strength paper, stickies, etc. ? Fillers and coatings such as clay, calcium carbonate and latex. ? Soluble material like starch. ? Fines and good fibers lost during processing

Although recycle paper processing systems are designed to remove detrimental contaminants without significant loss of desirable material, the rejects from screens, cleaners, flotation cells and washers usually contribute to the process loss of some good fibers. Frequent checks of the rejects to determine the amount of desirable material in the rejects is necessary to maintain a high yield. The yield should be kept as high as possible to reduce furnish cost and to avoid any unnecessary fiber losses, which will contribute to higher residue disposal costs. Therefore, yield calculation plays a major role in the economic viability of a mill.

The most important yield calculation is based on figures produced by accounting each month. Mill management uses these figures to evaluate plant performance and to compare yield with other mills using the same accounting system. This is often based on the total weight of the raw materials used in "as is" tons and on the net tons of paper produced on the paper machines as reported by production. The yield is determined by dividing output tons by input tons; however these figures are subject to frequent accounting adjustments due to for example:

? Incorrect scale weights ? Differences in monthly inventory of raw materials ? Discrepancies in accounting of paper machine broke, converting broke, rejected reels.

The accounting data does not show which raw material has the best yield to the paper machine head box. By combining laboratory data with accounting data it is possible to calculate the yield of each furnish component. The method will be described by calculating the yield of each furnish component in a hypothetical mill which uses 20% Sorted White Ledger (SWL), 20% Coated Book (CBK), 20% Newsprint (ONP) in the Deinking Plant plus 20% Wetlap pulp and 20% Broke at the papermachine. This hypothetical mill uses 30,000 "as is" tons of furnish per month and produces 21,000 "as is" tons per month As a result the losses are 9,000 tons per month and the yield becomes 70% as shown in Table 1, (which is part of an Excel spreadsheet designed for the hypothetical mill).

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Recovered Paper Yield Estimate Using Laboratory and Mill Accounting Data

Table 1: Yield based on "as is" tons in hypothetical mill (with spreadsheet variables in italics)

ATTRIBUTE

INPUT

OUTPUT

Deink Plant

Papermachine Furnish

PM

SWL CBK ONP Wetlap Broke Total

Prod.

Yield

"As is" Furnish Comp.

20% 20% 20% 20% 20% 100%

"As is" Tons/month

6,000 6,000 6,000 6,000 6,000 30,000

21,000

"As is"Losses 9,000

70.0%

The 50% moisture content of the Wetlap pulp is lowering the yield results. When the results are recalculated on a bone dry basis in Table 2 (assuming 6% moisture content for SWL and CBK, 9% for ONP and 7% for broke + production tons) the BD losses are 5,790 tons and the BD yield becomes 77.1%.

Table 2: Yield based on bone dry tons in hypothetical mill (with spreadsheet variables in italics)

ATTRIBUTE

INPUT

OUTPUT

Deink Plant

Papermachine Furnish

PM

SWL CBK ONP Wetlap Broke Total

Prod.

Yield

Moisture Content of "as is" Tons In Out Losses

6% 360 327 33

6% 360 327 33

9% 540 317 223

50% 3,000

174 2,826

7% 420 324 96

4,680 1,470 3,210

7% 1,470

BD Furnish Composition

22.3% 22.3% 21.6% 11.8% 22.0% 100%

BD Tons/month

5,640 5,640 5,460 3,000 5,580 25,320

BD Losses 5,790

19,530 77.1%

Calculating yield on a bone dry basis has eliminated 3,210 tons of moisture and reduced the losses from 9,000 to 5,790 tons per month. This improves the yield form 70% to 77.1%. The yield of each furnish component can be calculated by allocating losses using laboratory data.

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Recovered Paper Yield Estimate Using Laboratory and Mill Accounting Data

LABORATORY DATA FOR YIELD CALCULATION

In order to calculate the yield per grade, the laboratory data is needed on the following attributes of each furnish grade:

? 6 Cut Rejects or out throws from bale audits ? Solubles ? Britt Jar Fines & Ash (or Length weighted fines + ash at 550 C)

Normally these figures would be based on several samples from each furnish or even from each source. It will be necessary to assume some typical values for the hypothetical mill.

Table 3: Assumed Laboratory Data for Furnish and Product (with spreadsheet variables in italics)

ATTRIBUTE

INPUT

OUTPUT

Deink Plant

Papermachine

PM

SWL CBK ONP Wetlap Broke

Prod.

6 Cut Rejects

0.2% 0.4% 1.4% 0% 0.1%

0%

Solubles

4% 6% 1% 2.5% 1%

1%

Britt Jar Fines&Ash

26% 33% 27% 20% 11%

11%

It is also necessary to assume some arbitrary accounting data for residue disposal to landfill: ? 180 tons of bone dry coarse rejects (containing 30% good fiber from coarse screens). ? 4900 tons of bone dry residue (containing 20% good fiber from screens, cleaners, drains). ? Biological sludge is kept separate.

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Recovered Paper Yield Estimate Using Laboratory and Mill Accounting Data

CALCULATION OF BD LOSSES USING LAB DATA (6 cut Rejects + Solubles)

The amount of losses for each furnish component is calculated by multiplying the bone dry tons in Table 2 with the corresponding lab figure for each furnish component as shown in Table 3. For instance 5,640 tons of SWL contains 5,460 x 0.2% = 11 tons of 6 cut rejects + 3 tons fiber (due to 30% good fiber in coarse reject samples) = 15 tons of coarse rejects after rounding. The Wetlap pulp and PM Product contain no coarse rejects, so that no adjustment for coarse rejects in the finished product is required. The results calculated for coarse rejects and solubles based on laboratory results are shown in Table 4 and they are subsequently corrected for 180 tons "reported" losses in Table 5.

The solubles in the furnish minus the solubles in the finished product are assumed to go to the effluent plant. The solubles in the finished product are allocated according to bone dry furnish composition and are deducted from the solubles in each furnish component. The biological sludge produced from the solubles is kept separate in this example so that no adjustment in residue tons is needed for this example.

Table 4: Calculation of Losses for 6 Cut Rejects and Solubles Using Arbitrary Lab Data.

ATTRIBUTE

INPUT

Deink Plant

Papermachine

Net

SWL CBK ONP Wetlap Broke Losses

OUTPUT PM Prod.

6 Cut Rejects

In

11

23

76

-

6

Out

-

-

-

-

-

-

Losses

11

23

76

-

6

116

Fiber in 6 Cut Rej. 30%

3

7

23

-

2

35

Total

15

29

99

-

7

151

Solubles

In

226 338

55

75

56

Out

44

44

42

23

43

195

Losses

182 295

12

52

13

554

Table 5: Losses for 6 Cut Rejects and Solubles after Adjustment for 180 BD Tons "Reported" Losses

ATTRIBUTE

INPUT

OUTPUT

Deink Plant

Papermachine

Net

Residue

SWL CBK ONP Wetlap Broke Losses

BD Tons

6 Cut Rejects+Fiber Unaccounted 20%

15

29

99

-

3

6

19

-

7

151

1

29

Total

18

35 119

-

9

180

180

Solubles

182 295

12

52

13

554

Note: Figures in italics are normally based on laboratory and accounting data.

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Recovered Paper Yield Estimate Using Laboratory and Mill Accounting Data

CALCULATION OF BD LOSSES USING LAB DATA (BJ Fines & Ash)

The amount of BJ Fines & Ash in each furnish component is again calculated by multiplying the bone dry tons in Table 2 with the corresponding assumed or measured laboratory figure for each furnish component from Table 3. Some example calculations are shown below with the results summarized in Table 6:

? 5,640 BD tons of SWL x 26% = 1,466 tons BJ Fines & Ash ? 3,000 BD tons of Wetlap pulp x 20% = 600 tons of BJ Fines & Ash. ? 19,530 BD tons of finished product x 11% = 2,148 tons BJ Fines & Ash.

Since the Wetlap Pulp and Broke are added at the papermachine, it is necessary to assume 90% of the fines and ash will be retained in the product for this hypothetical example. (Normally this could be determined experimentally when the papermachine is running without any deinking plant furnish.) Since there are 2,148 tons of BJ Fines & Ash in the finished product, Table 6 shows that 1,092 tons came from the Wetlap Pulp + Broke, so that only 1,056 tons can come from the Deink Plant.

The 1056 BD tons of BJ Fines & Ash from the Deink Plant can now be allocated according to the bone dry furnish composition of the Deink Plant shown in Table 6 (33.7% SWL, 33.7% CBK + 32.6% ONP).

? 1,466 BD tons in SWL ? 356 tons in finished product = 1,111 tons of BJ Fines & Ash losses + 20% good fiber in the residue (222 tons) = 1,333 BD tons.

? 1861 BD tons in CBK ? 356 + 301 = 1807 BD tons ? 1474 BD tons in ONP ? 345 + 226 = 1356 BD tons

The results calculated for each furnish component in Table 6 (using assumed laboratory data) account for 4,641 tons of residue. Since the "reported" amount of residue determined by accounting equals 4,900 tons after moisture correction, the missing tons can be allocated proportionally to the amount of residue in each furnish component. This is done in Table 7.

Table 6: Calculation of Losses for BJ Fines & Ash

ATTRIBUTE

INPUT

Deink Plant

SWL CBK ONP

Papermachine Wetlap Broke

Net Losses

OUTPUT PM Prod.

Deink Furnish Comp.

33.7% 33.7% 32.6%

BJ Fines&Ash

In

1,466 1,861 1,474 600 614

Out:

Deink Out Alloc.

356 356 344

-

-

PM Retention 90%

540 552

Losses

1,111 1,505 1,130

60

61 3,867

Fiber in Residue 20%

222 301 226

12

12

773

Total

1,333 1,807 1,356

72

74 4,641

1,056 1,092 2,148

Table 7: Losses for BJ Fines & Ash after Adjustment for 4900 BD Tons "Reported" Residue

ATTRIBUTE

INPUT

OUTPUT

Deink Plant

Papermachine

Net

Residue

SWL CBK ONP Wetlap Broke Losses

BD Tons

BJ Residue + Fiber

1,333 1,807 1,356

72

74

Unaccounted 6%

74 101

76

4

4

Total

1,407 1,907 1,432

76

78

Note: Figures in italics are normally based on laboratory and accounting data.

4,641 259

4,900

4,900

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